This invention relates generally to opto-electronic receivers and more specifically to loss-of-signal detection by an opto-electronic receiver.
High speed communication systems often transmit data using lightwaves transmitted over fiber-optic cables. Opto-electronic receivers receive the transmitted data as lightwaves, and form corresponding electrical signals for use by computerized systems.
In the receiver, the lightwaves generally impinge on a photodetector generating a current. The current is generally provided to a transimpedance amplifier where it is translated to a voltage and amplified. As the current is often small, care must often be used in designing the transimpedance amplifier, and possible further amplifying stages, to obtain useable signals.
It is sometimes useful for an opto-electronic receiver to have the ability to detect a Loss-of-Signal (LOS) occurrence. A LOS may occur for a variety of reasons and causes. A laser source may fail, a fiber optic cable may be sliced, or various other problems may occur. In addition, even a partial loss of signal strength may be sufficient to result in degraded system operation when an optical source, such as an optical source coupled to the opto-electronic receiver via a fiber optic cable, is disconnected.
The small current generated by the photodetector presents difficulties in monitoring for LOS. The use of, for example, a peak detector, may require greater signal strength, and therefore the use of a signal some distance down the amplification stage. Use of such a signal, however, may result in erroneous LOS determination due to process, temperature, and other variations. Moreover, to avoid signal attenuation the components must often be placed close to the amplification stages, presenting layout difficulties.
The present invention provides a DC coupled Loss of Signal circuit. One embodiment of the invention comprises a loss of signal circuit. The loss of signal circuit is in an opto-electronic receiver, the opto-electronic receiver having a transimpedance amplifier AC coupled to a photodetector. The loss of signal circuit comprises a current-to-voltage circuit DC coupled to the photodetector, the current-to-voltage circuit receiving a DC current signal generated by the photodetector and providing a DC voltage signal. The loss of signal circuit further comprises a comparater coupled to the current-to-voltage circuit, the comparater receiving the DC voltage signal and providing a loss of signal signal.
In a further embodiment the invention comprises an opto-electronic receiver comprising a photodetector, a signal amplifier AC coupled to the photodetector, and a loss of signal circuit DC coupled to the photodetector. In one embodiment the signal amplifier comprises a transimpedance amplifier. In one embodiment the loss of signal circuit comprises a current to voltage circuit receiving a DC current signal and generating a DC voltage signal and a comparator comparing the DC voltage signal to a reference signal.
In a further embodiment the invention comprises an apparatus for generating a loss of signal signal for a photodetector circuit. The photodetector circuit is included in an opto-electronic receiver, with the photodetector circuit being AC coupled to an amplifier stage and the photodetector circuit generating a DC signal responsive to an optical signal. The apparatus comprises a current-to-voltage converter circuit receiving the DC signal from the photodetector circuit and generating a voltage signal in response to the DC signal, and a comparator circuit receiving a voltage signal from a current voltage converter circuit and generating an LOS signal. In a further embodiment the current-to-voltage converter further includes a current mirror circuit, a load resistor, and a voltage buffer circuit. In a further embodiment, the current to voltage converter circuit further includes a process and temperature compensator circuit.
In a further embodiment the invention comprises a method for generating a loss of signal signal for a photodetector circuit included in an opto-electronic receiver. In one embodiment the method comprises providing a current-to-voltage converter circuit preferably coupled to a photodetector circuit, providing a comparator circuit operably coupled to the current voltage converter circuit and receiving by the current-to-voltage converter circuit from the photodetector circuit a DC signal. The method further comprises generating by the current-to-voltage converter circuit a voltage signal in response to the DC signal, receiving by the comparator circuit the voltage signal, and generating by the comparator circuit an LOS signal.
In a further embodiment the invention comprises an apparatus for generating a loss of signal signal. The apparatus includes a current-to-voltage converter means coupled to the photodetector circuit for generating a voltage signal in response to a DC signal and comparator means coupled to the current-to-voltage converter means coupled to the current voltage converter means for generating a LOS signal in response to the voltage signal.